In spite of the fact that ultramafic and mafic plutonic rocks are of subordinate importance in the Earth’s crust structure, they are a very important source of information about the structure, properties, and material composition of the upper mantle and the Earth’s crust. These specific magmatic rocks allow reconstruction of the evolution of practically unobservable deep processes that have occurred for many hundreds of millions years in the upper Earth’s covers. These global processes and their products are studied using modern procedures for analysis of the trace-element composition of ultramafic and mafic rocks and the minerals composing them, including the studies of distribution regularities of REE in them. Data on the distribution of REE in ultramafic and mafic rocks collected in the last half of the century gave rise to the origin and development of many concepts and models of mantle magma formation, evolution of the upper mantle substrate and formation of various magmatic rocks, their massifs and complexes and ore mineralization occurring on the upper levels of the Earth’s crust available for direct observation. At the same time, despite achieved success in the geochemical study of ultramafic and mafic rocks and their minerals, and the rather wide application of new developed parameters and criteria for solving different problems in the systematization, metallogeny, and petrology of these formations, the information potential of rare earth and other trace elements is far from being exhausted.